The proposed research includes several problems. The first is a study of the relations between torsion fractures and microscopic structure of adult human cortical bone. Femoral specimens (415) whose torsional properties are known are available for histological analysis. Enlarged prints of photomicrographs of cross sections of the specimens close to the fracture site will be analyzed with respect to the percentage of the cross section formed by osteons, osteon remnants, interstitial lamellae and spaces (as seen in ordinary light microscopy); by osteons, etc. with different collagen fiber orientation (as seen by polarized light); and by osteons, etc. with different degrees of calcification (as seen by microradiography). A second project is determination of fracture characteristics of bone of a single histological type. The histological structure of cross sections of tibias of one side will be the basis for selection of test specimens from the opposite bone. A third project is the determination of fracture characteristics in relation to histological structure of specimens from the cortex of the major long bones of a 14-year-old boy. Special attention will be given to the epiphyseal region which is frequently injured in teenagers. Current studies on mechanical properties of histology of bone in severe endemic fluorosis will be completed. Mechanical properties will be determined by approved materials testing techniques and analyzed for statistical significance on computers.